The present invention relates to radio frequency receivers such as those used in wireless alarm systems in communication with a plurality of alarm sensors, and in particular radio frequency receivers comprising an interference detection circuit capable of detecting continuous and non-continuous interference.
Wireless security systems operating in residential and commercial buildings are often relied upon for "Safety of Life" applications where an alarm condition may signify a health or security emergency. Many national regulatory agencies place stringent requirements on the operation of these types of systems. For instance, in the United States Underwriter's Laboratory issues specifications, in the United Kingdom British Standards apply and in most of Europe CENELEC harmonized "norms" provide standardized requirements.
These specifications all include a requirement for the protection of system integrity against interference and jamming in the band of operation. Typically, it is required that an interference or trouble condition be indicated when a Continuous Wave (CW) Interferer or continuous interference source operates at a given power level (e.g., eight decibels above the sensitivity of the receiver) for a specified duration of time (e.g., for thirty seconds during a one minute duration or more than ten seconds). Likewise, an interference or trouble condition must be indicated when a non-continuous interference source, such as a stream of toggling data (e.g., 101010101010 . . . ) at the characteristic data rate of the system, persists for a specified duration of time (e.g. more than ten seconds) at a given power level (e.g., eight decibels above the sensitivity of the receiver).
Many low-cost systems are optimized for operation using Manchester encoded data to amplitude modulate (AM) wireless transmissions. At a corresponding receiver, the demodulated input is ac-coupled to block dc levels using, for instance, a capacitor in series between the antenna output and the receiver input. This is a particularly efficient method for decoding Manchester encoded data streams. However, the problem with ac-coupling is that it blocks continuous interference (which appears as a dc level upon demodulation) as well and, ideally, the system should detect the presence of continuous interferers and act accordingly. Another problem with ac-coupling is that despite the careful choice of capacitive values to match the data rate of the wireless transmission system, the resulting decay time of the ac-coupling may still be too fast or too slow to pass a particular non-continuous interference data stream.
Therefore, it would be advantageous if a communication system was suitable for use with alarm systems while providing means for effectively detecting continuous as well as non-continuous interference. In addition, such a system should provide a low cost solution to the interference detection problem that is readily adaptable to various national requirements specifying different interference parameters and patterns.